Human body communication is a technology for transmitting signals between devices connected to a human body, using the conductive human body as a communication channel. A communication network between various portable devices, such as personal digital assistants (PDAs), portable personal computers, digital cameras, MP3 players, mobile phones, etc. and stationary devices, such as printers, televisions, entrance systems, etc. can be formed by a user simply contacting each of the devices.
A conventional human body communication technology realizes low speed data communication at a maximum speed of several tens of Kbps using a specific frequency, such as frequency shift keying (FSK) or amplitude shift keying (ASK). An application field of the conventional human body communication technology is limited to a field that requires simple data transmission.
A technology for directly applying a digital signal, i.e., a non return to zero (NRZ) signal, to a human body and restoring a received signal using a photoelectric effect realizes 10 Mbps communication by remarkably improving transmission speed.
FIG. 1 is a diagram illustrating a conventional communication apparatus which directly applies an NRZ signal to a human body and electrically restores an NRZ signal.
The conventional communication apparatus includes an NRZ data transmitter 100 and a clock and data recovery (CDR) based receiver 110.
A wideband pulse signal received through a human body channel is amplified by a preamplifier and 0 and 1 states are restored by a trigger.
An amplitude value of the restored signal is down to a ground level by a level shifter and then the restored signal is inputted into the CDR.
FIGS. 2A and 2B are diagrams illustrating another conventional communication apparatus which directly applies an NRZ signal to a human body and restores an NRZ signal using an electro-optic (EO) effect.
An electric field generated by a human body changes a refractive index of an EO crystal 210 illustrated in FIG. 2B.
The polarization of the light emitted from a laser diode 220 is varied with the change of the refractive index of the EO crystal.
The variation of the polarization of the light results in the electric power change which is measured by a polarizing beam splitter 230 and two photo detectors (PDs) 240.
Such high speed data transmission widens the application fields of conventional communication apparatuses, which has been limited to a simple data transmission, to surroundings of life.
Despite such improvement in the communication speed, the technology of using the photoelectric effect is unable to be applied in small devices due to technical problems, such as the size of a module, power consumption, etc.
In order to solve the above problems, technologies of using an electrical restoration have been introduced. These technologies not only realize 2 Mbps communication but can also be realized in a single chip that can be applied in various electronic devices.
Such technologies can be used in realizing a network between various sensors that can be applied to a human body, such as electrocardiographs (ECGs), non-invasive blood pressures (NIBPs), heart monitors, etc, and also a human body based network, such as a wearable computer, which was difficult to be realized.
A human body is formed of various materials in various forms, and can function as an antenna in a wide frequency domain due to its conductivity and high dielectric constant.
Based on the above properties, the human body can be used as an antenna for communication, but the human body may be exposed to a signal that is not desired from an external electronic device.
Frequency modulation, such as FSK, ASK, or phase shift keying (PSK) can select and use a domain having low exterior interference.
In realizing communication, low signal-to-noise ratio may be requested or interference may be difficult to be resolved.
A method of directly transmitting a digital signal requires a very wide bandwidth, and the digital signal may be received from a transmitter after having been exposed to strong interference from peripheral devices.
When interference is generated in a signal bandwidth, a desired signal is unable to be properly separated even if the reception of a receiver is good.
Also, in order to directly transmit a digital signal at a rate of several Mbps, a bandwidth of over several tens of MHz is required. When this digital signal is applied to a human body, a frequency of a signal greater than a certain frequency is emitted. Accordingly when there are several users, interference between users occurs even though they are not in contact. Thus, a stable network cannot be realized.